Rotational angle sensors for non-contact sensing of rotations are increasingly used particularly in automobile technology. Rotational angle sensors on the basis of giant magneto resistance (GMR effect) according to the spin valve principle have several advantages as compared to the AMR sensors (sensors using the anisotropic magneto resistance; AMR: anisotropic magneto resistance) currently used. For example, rotational angle sensors on the basis of the GMR effect have an inherent 360° unambiguousness, when a bridge assembly is used, as well as a significantly higher sensitivity than common AMR sensors. Therefore, the use of rotational angle sensors on the basis of the GMR effect may involve both performance advantages and cost advantages.
Basically, spin valve systems are constructed of two ferromagnetic layers separated by a non-magnetic intermediate layer. The direction of the magnetization of one of the two ferromagnetic layers is pinned by coupling to a magnetically hard anti-ferromagnetic layer. The strength of this coupling is described by the so-called exchange bias field. The other ferromagnetic layer, i.e. the layer whose magnetization is not pinned, is free to move its magnetization in an external magnetic field so that the angle between the magnetizations of the pinned and the free layers changes in a rotating external magnetic field. As, with giant magneto resistance (GMR effect), an electric resistance of a layer system depends on the mentioned angle, the angle may be concluded by means of resistance measurement.
In order to realize a 360° detection by means of spin valve GMR/TMR structures (TMR: tunnel magneto resistance), several layer systems are connected to form two Wheatstone bridges. In this way, a maximum signal may be achieved. One of the bridges has reference magnetizations perpendicular to the reference magnetizations of the other bridge. Within each of the two bridges, the reference magnetizations are arranged anti-parallel. Thus, both bridges provide sinusoidal main signals depending on the rotational angle of an external magnetic field which (ideally) are phase-shifted by 90° with respect to each other. Hereinafter, the two main signals are also referred to as sine main signal and cosine main signal.
As, due to process reasons, individual signals and/or measurement signals from the two bridges may differ in amplitude, may be provided with an offset and may exhibit a deviation of orthogonality, the measurement signals are first calibrated/compensated. Thereafter, the angle to be ascertained may be determined via an arctan calculation of the two signals.
For ideally sinusoidal measurement signals from the Wheatstone bridges, the described calculation would provide the accurate angle. In reality, however, it can be seen that there is still a residual angle error related to a measurement angle even after offset, amplitude and orthogonality calibrations/compensations. The residual angle error includes so called anisotropy errors and hysteresis errors.
It has been shown that the anisotropy error is typically about three to five times the hysteresis error. For a highly accurate angle measurement having an angle error of less than 1° over a sufficient field and temperature range, it is necessary to reduce the largest angle error contribution remaining after offset, amplitude and orthogonality calibration/compensation, i.e. the anisotropy error.
The term anisotropy error includes all errors remaining after offset, amplitude and orthogonality calibration. The mentioned errors include contributions due to excessively high magnetic anisotropy, for example a form anisotropy due to strip width of the sensor elements and growth or process-induced uniaxial magnetic anisotropies. The errors also include deviations caused by slight movements of the pinned layer as well as the reference layer in the external magnetic field. Such movements of the pinned layer and the reference layer indicate insufficient anisotropy.
Angle errors depending on the output angle from which a certain angle is approached are referred to as hysteresis errors.
As the anisotropy error comprises a dominant part of the whole residual angle error, a compensation or reduction of the anisotropy error may lead to a significant improvement of sensor accuracy.
German patent application DE 101 54 153 A1 shows a method for offset calibration of angle sensors determining the angle to be measured on the basis of a corresponding sinusoidal and cosine signal. For the improvement of measurement accuracy of an angle sensor, a sample of the sinusoidal signal is taken when the cosine signal has a zero crossing, and a sample of the cosine signal is taken when the sinusoidal signal has a zero crossing. An offset is determined by averaging opposing samples. Furthermore, DE 101 54 153 teaches to take at least three pairs of measurement values by determining the sinusoidal signal and the cosine signal for at least three different angles. Thereupon an error square adaptation to a hypothetical circle may be performed for these measurement values, thus allowing to determining the offset.
German patent application DE 101 54 154 A1 shows a method for the compensation of an offset drift of an angle sensor and an angle sensor with an offset compensation. The angle sensor determines an angle to be measured on the basis of a corresponding sinusoidal signal and a cosine signal. The above reference teaches to first ascertain an amplitude of the cosine signal and/or the sinusoidal signal and to determine an associated offset value on the basis of the ascertained amplitude value. This allows to correct the sinusoidal and cosine signals generated by the goniometer, thus also allowing to determine a measurement value for the angle with improved accuracy at the same time.
German patent application DE 101 48 918 A1 describes a method for the offset compensation of a magnetoresistive path or angle measurement system. The path or angle measurement system includes at least one Wheatstone bridge having four locally staggered magnetoresistive resistors provided with DC voltage. A permanent magnet is moved past the magnetoresistive resistors, the respective bridge voltage being measured. According to the above reference, a calibrating cycle is performed when starting up the measurement system by moving the permanent magnet past the at least one bridge for a measurement period. Extremes of the bridge voltages measured in that way are evaluated by forming their average values. Then corresponding compensation values are formed from the above average values as offset values to be taken into account by the measurement system. The compensation values are then stored in a register.
German patent application DE 100 52 609 A1 describes a method for the compensation of an offset drift of a goniometer. According to the above reference, a goniometer is located on a rotatable shaft and comprises a generator scanned by an associated sensor element. According to the rotational angle of the rotatable shaft, a sinusoidal or cosine signal is provided to an evaluating unit. For the compensation of a temperature offset, a third order polynomial is used. In addition, averaging is performed for correcting a long-term drift. This allows to gradually correct a long-term drift. If the long-term drift exceeds a given limit, a diagnostic function helps to detect this error and to output a corresponding message.
German patent application DE 101 30 988 A1 describes an angle sensor and a method for adjusting an angle sensor. According to the above reference, the angle sensor provides two measurement voltages whose amplitude courses plotted in a plane form a circle. The angle sensor further includes an evaluating circuit for evaluating the measurement voltages dropping at the bridge circuits during an angle measurement. The evaluating circuit is provided for a calculation and a correction of a measured angle and provides an output signal for further processing of the calculated and corrected angle. The evaluating circuit supplies a correction quantity to the output signal for the correct adjustment of the angle calculated from the measurement voltages. The evaluating circuit calculates the correction quantity from the Cartesian coordinates of a center of a circle on the circumference of which there are measurement voltages of an angle measurement.
German patent application DE 101 63 528 A1 describes a method for error compensation of sine/cosine position measurement systems, wherein offset, amplitude and phase errors may be corrected. An offset, amplitude and phase error of a sine and cosine track is determined from measurement values. Thus, offset, amplitude and phase errors may be compensated for an evaluation of a fine position. When performing the method, there is further a temperature compensation of error parameters.
The German patent application having the application number 10 2004 024 398 A1 describes a method and a device for adjusting a determination rule of an angle sensor. The above patent application describes a method for adjusting a determination rule for an error compensation of an angle sensor designed to detect a first component oriented along a first axis and a second component oriented along a second axis. Based on the first component and the second component, an angle is then determined according to the determination rule. For a calculation of offset and/or amplitude and/or axis angle errors, component pairs consisting of values of a first and a second predetermined component are detected. According to a first embodiment, component pairs are inserted into an ellipse equation system, whereupon at least one ellipse coefficient is determined from the ellipse equation system. The determination rule is then adjusted depending on the one determined ellipse coefficient or the plurality of determined ellipse coefficients. In a further embodiment, extremes or zero points are used for a determination of the offset and/or amplitude and/or axis angle errors.
It is apparent from the above prior art that current magnetic field sensors and/or rotational angle sensors using the anisotropic magnetoresistive effect (abbreviated as AMR sensors) do not provide any correction for the anisotropy error because the anisotropy error is not important in AMR sensors. In sensors using the giant magnetoresistive effect (abbreviated as GMR sensors), no effective correction of the anisotropy error is known either.
Influencing the causes for anisotropy errors is difficult and results in large technical effort and has further disadvantages. In principle, it is possible to achieve a limited reduction of an anisotropy error due to excessively high anisotropy of the sensor layer by influencing the causes. For example, the so-called form anisotropy may be reduced by increasing a width of the GMR resistance structures. However, this results in an increased area need, because, for reasons of limited current consumption, resistance for the GMR resistors may not drop below a certain level. However, a larger area need for the GMR resistors opposes a cost-optimized realization of magnetic circuits and is thus not advantageous. An increase of coupling strengths of the reference magnetic system as a possible measure against the case of too little anisotropy, however, is nearly impossible due to given material properties.
Thus, a measurement method is conventionally used in angle measurement with GMR resistors and/or GMR elements that is not able to compensate anisotropy errors.
According to prior art, an angle measurement using GMR resistors and/or GMR elements includes determining calibration parameters for signal main components (sine main component, cosine main component) as a part of an angle sensor calibration and calibrating/compensating the measured main components (sine main component, cosine main component) in an angle measurement in the sensor arrangement. When determining the calibration parameters for signal main components, a first step comprises taking measurement data for measurement angles in the angle range between 0° and 360°, i.e. for a full revolution. Values of the two main components (ideally of a main sine signal and a main cosine signal) are taken for a plurality of angles. Then a sinusoidal signal is respectively fitted to the course of the main components of the measurement signal (i.e. the main sine signal and the main cosine signal). For this purpose, one of the known methods for curve fit may be used. In this way, an offset (main offset) and an amplitude (main amplitude) may be determined for each of the two main signals (main sine signal, main cosine signal). Furthermore, a potential deviation with respect to an orthogonality between the main sine signal and the main cosine signal may be determined. Thus, the parameters of two sinusoidal curves are known which approximate the main signals provided by two angularly offset sensors.
The gained calibration parameters (i.e. of the main offset and the main amplitude for the two main signals and of a parameter for an orthogonality correction) are then used in an evaluation of an angle measurement in the sensor arrangement. An angle measurement includes a calibration/compensation of the measured main components and starts with a measurement of the individual components (main components) provided by the sensor, i.e. the main sine signal and the main cosine signal. In a further step, the main components are calibrated, wherein there is a calibration of offset, amplitude and orthogonality. Here, the calibration parameters determined in a calibration, i.e. for example the main offset parameters, the main amplitude parameters and an orthogonality parameter, are used. With the help of the calibrated components, an angle to be determined and/or an angle position of an external magnetic field to be calculated is then calculated.
If a sensor is operated in calibration conditions, i.e. in the external conditions in which the calibration parameters were determined, an overall residual error after the described compensation, i.e. after a calibration of the main components with regard to offset, amplitude and orthogonality, is composed of the anisotropy error and the hysteresis error. The share of the anisotropy error in the overall residual error is typically about 80%.
Thus it can be said that a noticeable reduction of an anisotropy share in the angle error may contribute to a significant increase in sensor accuracy. However, according to prior art, no device and no method is known that allows a significant reduction of the anisotropy error in a rotational angle sensor in a way realizable in a technically advantageous manner.
Summarizing, prior art does not know any satisfying solution to compensate anisotropy errors in a magnetoresistive angle sensor. The only thing known are merely devices and methods for compensating offset errors, amplitude errors and phase errors.